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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Understanding the significance of the tropospheric hot spot

What the science says...

Satellite measurements match model results apart from in the tropics. There is uncertainty with the tropic data due to how various teams correct for satellite drift. The U.S. Climate Change Science Program conclude the discrepancy is most likely due to data errors.

Climate Myth...

There's no tropospheric hot spot
The IPCC confirms that computer modeling predicts the existence of a tropical, mid-troposphere “hot spot” about 10km above the Earth’s surface. Yet in the observed record of the Hadley Centre’s radiosondes, the predicted “hot-spot” signature of anthropogenic greenhouse warming is entirely absent (source: Christopher Monckton)

Part 1: The “Hotspot” as an Alleged Fingerprint of Anthropogenic Warming

A great deal of the confusion surrounding the issue of temperature trends in the upper troposphere comes from the mistaken belief that the presence or lack of amplification of surface warming in the upper troposphere has some bearing on the attribution of global warming to man-made causes.

It does not.

Attribution of anthropogenic origins of the current climatic changes can be tested from many different directions. On of the most clear examples for those with some familiarity with the Earth’s atmosphere is the issue of stratospheric cooling. If the sun were to suddenly increase its output by 2%, we would rightfully expect the atmosphere as well as the surface to warm up in response. This can be examined, for instance, by looking at the response in a GCM like GISS ModelE:

2% increase in solar forcing (via RealClimate)

Likewise, if we were to double preindustrial levels of CO2, we would expect the surface and the lower atmosphere to warm. However, unlike the case of increasing solar influence, we would not expect the lower atmosphere to warm through at all levels. Increasing the greenhouse effect should warm the surface and troposphere, but cool the lower stratosphere.

Doubling of CO2 (via RealClimate)

In the doubled CO2 scenario, there is a pronounced cooling of higher altitudes, i.e. the stratosphere, and this feature is entirely absent in the +2% solar scenario.

This stratospheric cooling is a fingerprint of increased greenhouse (as opposed to solar) warming. For a more in depth discussion of why the stratosphere cools under enhanced greenhouse warming, see discussions at Skeptical Science and The Science of Doom. In other words, the difference in the two simulations is not the presence of a "hot spot" in one and its absence in the other, it's the stratospheric cooling apparent in the increased CO2 simulation.

In the IPCC Fourth Assessment Report (AR4), historical forcings were simulated in the Parallel Climate Model, and and the zonal mean temperature responses to each were broken out in separate panels. There was some increase in solar irradiance during the period, which shows up as a modest amount of warming throughout the atmosphere, with some amplification in the upper troposphere (the sort of greenish-yellow and yellow patterns respectively in panel a). As we all know, there was a significant change in GHG forcing during that time, which manifests as surface warming, amplified upper troposphere warming, and stratospheric cooling (panel c), and the net effect of all forcings was shown (panel f).

So far so good. Right? Well, this is actually where things went off the rails.

Climate “skeptics” apparently became convinced that the “hot spot” in Figure 9.1c was the fingerprint of anthropogenic warming the IPCC was referring to, rather than stratospheric cooling coupled with tropospheric warming.

As he so often does, Monckton serves as a useful example of getting things wrong, claiming:

the models predict that if and only if Man is the cause of warming, the tropical upper air, six miles above the ground, should warm up to thrice as fast as the surface, but this tropical upper-troposphere “hot-spot” has not been observed...

This unequivocally incorrect claim was also made in the NIPCC "skeptic" report (Section 3.4), which was signed off on by such supposedly "serious" contrarians as CraigIdso and S.FredSinger.

The mistaken belief in “skeptic” circles is that the existence of anthropogenic warming somehow hinges on the existence of the tropospheric “hot spot”- it does not. Period. Tropospheric amplification of warming with altitude is the predicted response to increasing radiative forcing from natural sources, such as an increase in solar irradiance, as well. Stratospheric cooling is the real "fingerprint" of enhanced greenhouse vs. natural (e.g. increased solar) warming.

Part 2: The Existence of Amplified Warming in the Upper Tropical Troposphere

So, does the “hot spot” actually exist? That is to say, is the tropsosphere actually warming as expected? Unfortunately, the answer to this is much less cut and dry.

There is a good theoretical basis for this expectation of amplification in the upper troposphere relative to the surface. We expect that an increase in radiative forcing would result in a moist adiabatic amplification of warming with altitude, i.e. that the troposphere would warm faster with height. This also appears as an emergent property in climate models, which show a similar vertical profile of warming to that expected under the moist adiabatic lapse rate.

Unfortunately, actually determining what is happening in the real tropical troposphere has proven to be quite difficult. Perhaps the largest reason for this is the quality of data from the main source of our information from this region for long time periods- radiosonde networks.

Although on seasonal and annual scales, some radiosonde records are in relatively good agreement with theoretical and modeling expectations, on decadal timescales, they show less warming or even cooling of the upper troposphere. However, the tropics, especially at higher altitudes, are a notorious problem area for most if not all of the older radiosonde networks. And attempts to stitch together longer records from multiple networks (and integrate them with newer satellite records) have introduced problems as well. There have been many attempts to quantify and remove these biases (e.g. Randel 2006, Sherwood 2008). Although these attempts have managed to reconcile the observational data with theoretical and model expectations within overlapping uncertainty intervals, the real world behavior of the troposphere is still unclear (Bengtsson 2009, Thorne 2010).

Allen and Sherwood sought to side step the problems associated with the radiosonde data entirely, and examined the “dynamical relationship known as the thermal-wind equation, which relates horizontal temperature gradients to wind shear”. Thermal wind speed data, in contrast to the temperature data, lacked many of the systematic adjustment issues and other errors, and were used as a proxy for temperature. Allen and Sherwood found that the troposphere appeared to be warming in reasonable agreement to theoretical and modeling expectations.

Vertical profile of tropical mean temperature trends. Trends reflect the mean change in temperature (in K per decade) between 20° N and 20° S for the period 1979–2005, obtained from radiosonde temperature measurements5 (blue and green colours), climate models8 (dashed orange, with grey shading indicating 2-sigma range) and the new reconstructions from radiosonde winds4 (pink, with error bars indicating 2-sigma range). The surface temperature change11 from 1979–2005 (grey asterisk) and the vertical profile inferred from the moist adiabatic lapse rate (dashed yellow) are also shown. The model range was derived by scaling the model vertical trend behaviour (which has been shown to be tightly constrained8) and its uncertainties8 by the surface trend. Prior to 2007, only the HadAT and RATPAC estimates existed, and a case could be made for a fundamental discrepancy between modelled and radiosonde observed behaviour. (Thorne 2008)

Recently, Johnson and Xie have approached the question from a different but similarly indirect angle. They examined trends in tropical sea surface temperatures (SSTs) and precipitation, which have direct implications for the behavior of the vertical tropical tropospheric temperature profile:

As the SST threshold for convection is tied to convective instability, this threshold must be strongly related to the tropical upper-tropospheric temperature. Observations show that tropospheric temperatures in the tropics approximately follow a moist-adiabatic temperature profile, which suggests an adjustment of upper-tropospheric temperatures in response to surface temperatures in the tropics. This hypothesis of moist-adiabatic lapse rate (MALR) adjustment predicts a close covariability between the SST threshold and tropical mean SST. If true, the variability and long-term trend of the SST threshold may reveal important information about the variability and trends in the tropical troposphere.

Climate warming over the tropical oceans [exaggerated]: a) In a climate before warming, convection and heavy tropical rain is restricted to a region where SSTs exceed a threshold value (dotted line), and temperatures decrease with altitude. b) Johnson and Xie show that this SST threshold has risen in tandem with mean SSTs over past decades, and that the area of surface ocean where convection occurs has remained constant. As a result of warming at the sea surface, air temperatures rise most at high altitudes. (Sobel 2010)

Tropical convection and thus precipitation is heavily dependent on sea surface temperatures (SSTs). Thus the absence of increased precipitation is indicative of stability upwards through the troposphere, which suggests that the upper tropical troposphere is indeed warming faster than surface temperatures.

[T]he similarity between the trends of SST and the SST threshold for convection in [the following figure] is consistent with approximate MALR adjustment in observations and inconsistent with reduced upper tropospheric warming relative to the surface, as indicated in some observational data sets. Although the statistical uncertainty of 30-year trends is rather high, the clean relationship between the SST threshold and tropical mean SST at all timescales in both observations and models increases confidence that the tropical atmosphere is warming in a manner that is broadly consistent with theoretical MALR expectations.

Time series of tropical mean SST and the SST threshold for convection. Thirty-year time series of annual tropical mean (20° S to 20° N) SST (black diamonds) and two estimates of the SST threshold for convection (blue squares and red stars). Linear trend lines are also shown. The linear trends with 95% confidence intervals for the tropical mean SST, the PD2mmd^-1 SST threshold estimate and the linear P fit SST threshold estimate are 0:088±0:057;0:083±0:076 and 0:080±0:113 °C per decade, respectively. The effective degrees of freedom in the 95% confidence interval calculations account for the lag-1 autocorrelation in the residual time series. (Johnson 2010)

Is this the “final word” on amplified tropospheric warming? Of course not. Ideally, instrumental biases and gaps in the satellite and radiosonde records can be sorted out, longer records from newer networks will provide more confident results, and we can get an even clearer picture of what’s going on in the tropical troposphere. In the meantime, however, this is further evidence that things are behaving more or less as we’d expect.

But moreover, these papers illustrate some key aspects of science (and particularly climate science), that could use some emphasizing. Science is iterative, not dictative or supernaturally revelatory. There’s no single, infallible decree. Science is the process by which we strive to best approximate reality. The first results are not necessarily the “best” results, and they certainly are not written in stone. Our monitoring systems, particularly (ironically?) the ones with multidecadal records, were not designed for the kind of questions we may be trying to investigate with them. Or, to paraphrase a certain former Secretary of Defense, you study the world with the instruments you have, not the instruments you might want or wish to have at a later time. Would life be a lot easier if we had designed and implemented global climatic monitoring systems in the 60s and 70s with an eye for explicitly addressing the questions we have now? Of course! But we have to make do with what we’ve got, and that means working with problematic data and finding creative ways to work around them. To that end, it’s worth pointing out, proxies aren’t just used to study the past. Through comments here and at other blogs, I get the impression that people think using proxies is restricted to paleo questions. The presumption seems to be that in our digital, high-speed, satellite-monitored age, direct observations are the only game in town. As this case shows, however, this is decidedly not true. Indirect methods of assessing an issue are sometimes the only (or only alternate in the case of suspect data) methods available. And that’s not necessarily a bad thing! Sometimes looking at a question from a different angle can avoid some potential complications altogether. And finally, there is a pernicious lie that can be heard in climate denialist circles, typified by remarks like these from Dick Lindzen:

[I]t has become standard in climate science that data in contradiction to alarmism is inevitably ‘corrected’ to bring it closer to alarming models. None of us would argue that this data is perfect, and the corrections are often plausible. What is implausible is that the ‘corrections’ should always bring the data closer to models.

Lindzen’s implication is clear- observational data that don’t support “models” are fraudulently adjusted until they do, ergo climate change is at least partially an artifact of data manipulation. This is, in a word, absurd. First, due to the ludicrous nature of the claim and its inherent absolutism, it’s easily debunked by a single contrary example. Take, for instance, the notorious problem of climate models producing double ITCZs (e.g. Zhang 2006). This is a case in which models produced a result at odds with both theoretical expectations and observations. No one attempted to claim that the models were right about this and theory and observations were wrong.

This does illustrate a germ of truth buried in Lindzen’s conspiratorial falsehood, however. Climate models and theoretical climate dynamics/meteorology are constrained by physics, and for the most part, models tend to agree with physics-based, theoretical underpinnings of meteorology/climate dynamics. When there is an apparent discrepancy between “models” and observations, that often (but not always) means there is a discrepancy between general, theoretical meteorological expectations and the observational data. It’s not a case of trying to reconcile the observations with climate models, but rather trying to reconcile observational data (which often have well known biases) with our physics-based understanding of the climate system.

When people are quick to point out some alleged contradiction between climate models and a data set, they don’t realize that often as not they are pointing out a contradiction between the observations and our fundamental explanations of the climate system irrespective of the question of anthropogenic influence. And far from justifying a position of “nothing to worry about”, significant flaws in our understanding of the climate system would greatly strengthen the case for mitigation from a risk management perspective, as uncertainty and ignorance of consequences increase the relative value of insurance. But that’s a topic for a different day…

Comments

It is also worth noting that the claims that "hot spot" is a signature of AGW is not correct, at least in the sense of it being a distinguishing characteristic for that particular warming mechanism. As Gavin Schmidt has shown here, the same sort of signature would be expected if the warming were due to an increase in solar irradiance: http://www.realclimate.org/index.php/archives/2007/12/tropical-troposphere-trends/ [Where the two mechanisms (greenhouse gases vs solar) are distinguishable is in the stratosphere where solar should cause warming and greenhouse gases should cause cooling. There, the satellite and radiosonde data unambiguously show cooling.]

In fact, this amplification of trends as you go up in the tropical troposphere is predicated on a very basic piece of physics, what is called "moist adiabatic lapse rate theory"...And, it is expected to hold not only for the multidecadal trends but also for the temperature fluctuations over shorter intervals (say on the order of months to a year or so) due to things like El Nino oscillations. And, the satellite and radiosonde data confirm this amplification for the fluctuations (see Santer et al., 2005: http://www.sciencemag.org/cgi/content/abstract/sci;309/5740/1551 ).

Where the satellite data and radiosonde data do not necessarily confirm the amplification is for the multidecadal trends...but both the satellite data and radiosondes are known to have problems that can easily produce spurious secular trends over these long time periods. So, in other words, the data only deviates from the theoretical expectations where the data is least reliable (and whether it deviates significantly depends on whose satellite analysis or radiosonde analysis or re-analysis you believe).

This whole thing reminds me of the "God of the Gaps" argument. Originally, the UAH analysis of the satellite data showed the troposphere to be cooling globally...in contradiction to the surface record...and this was a major "skeptic" talking point. Then, a longer record and corrections for problems in the analysis (like the neglect of orbital decay of the satellites) turned the global cooling trend into a global warming trend, but it wasn't as strong as at the surface...and this was still a major "skeptic" talking point. Now that a still longer record and further corrections (along with a completely independent analysis of the satellite record by the RSS group) show that the trend found by satellites matches the trend found at the surface globally within error bars, the "skeptic" talking point has become the trend in the tropics. And, no doubt, when that is resolved, they will find something else to point to! No matter how little the "gaps" are, there is always room to say, "The evidence disproves AGW!"

You must have changed your spots, posting at a real science web site like SkS ;)

This thread is about temperatures in the upper tropical troposphere, not the stratosphere. Your questions are on the wrong thread. Try here. The search engine on the top LHS of the page is very useful for finding the relevant thread, as I just did.

I would help answer your questions, I have a paper in mind, but I do not wish to embark on a fool's errand by engaging you given your history at BBC and WUWT.

apologies albatross but the article does talk about cooling in the stratosphere - i'll try the other thread

didn't know i was being watched so carefully! what nake do you post under at the BBC and WUWT?

just goes to prove that i do read the opposite view and links that people suggest, and i really am open to other views. A little shame that others on the BBC website are less prepared to look at both sides of the argument

This is not WUWT or BH. So no need to be paranoid, and please stop alluding to nefarious conduct by me. I have seen your posts at WUWT and on Richard Black's "skeptic" riddled blog, simple as that. So pardon my prudence.

Re "stratospheric cooling". My apologies, I was looking at the "intermediate" page, not the "advanced page and reference is made to stratospheric cooling in the latter. Your questions about the stratosphere are more suited and more likely to be answered on the other page though, and I see you have posted there. Thanks.

MangoChutney, I would be interested to know if you have read the 'Advanced' tab of this post. If you have, are you prepared to accept that your past assertions on Richard Black's blog, and I'm sure in many other places, tht the tropical tropospheric hotspot is an 'AGW signature that hasn't been found', is incorrect? Actually your assertions at the BBC on this thread have been almost a carbon copy of Monckton's quote above.

So, will you do what few skeptics have the strength of character to do, and accept that you were wrong on this point?

#10 Mango, you like taking the emotive line, but I am after a scientific reasoning as to why you believe that the tropospheric hotspot is specifically an AGW signature. The 'advanced' tab clearly shows that it is by no means unique to AGW, so where do you think this is wrong, and upon what scientific basis?

You have, elsewhere, insinuated that data has then been massaged to fit the models (much like Lindzen suggested as discussed above), yet clearly this is not the case. This area of the science is one that has significant uncertainties (as the Thorne graph above shows), yet is not fundamental to the validity of our understanding of the climate system. Lindzen, Monckton and others love to portray otherwise, and I'd hate to see you be taken in by their misinformation.

Forget the emotion, or feeling hurt that you've been called out on these statements you've made, as you now have a great chance to present the science for your case.

Nobody has asked you to confess your sins, can you please tone down the rhetoric and stop arguing strawmen. IIRC, Skywatcher was asking if you accept that you are wrong about the so-called hot spot in the upper tropical troposphere being purely an anthropogenic signal/signature/fingerprint.

So what are you not convinced about in this regard?

1) That the hot spot is not purely an anthropogenic "signal"? It is not a fingerprint of AGW, it is a fingerprint of a warming planet regardless of the cause of the warming.

2) That the "hot spot" upper tropical troposphere not discernible in the observations?

Please state your position clearly. This is exactly why I asked you on another thread to list your top three or so primary concerns, to avoid what is happening now.

Now concerning the validation of the model simulations of the "hot spot". I suggest that you read the very latest literature on this by Thorne et al. (2011). They conclude that:

"It is concluded that there is no reasonable evidence of a fundamental disagreement between tropospheric temperature trends from models and observations when uncertainties in both are treated comprehensively."

If that paper in addition to the other excellent papers cited in the main post do not convince you then nothing will I'm afraid. We can provide you with facts, but the rest is up to you.

"thing as a graph showing the actual observed data"

What is wrong with the data presented by Thorne et al. (2008)?

Are you suggesting that the data from multiple agencies and data observation platforms are not trustworthy? And why do both "sides" have to agree on hard, cold facts? You have a very troubling idea of how science works Mango.

I think the argument is not that the tropospheric hot spot is unique to co2 forcing. Instead, I believe what is at question is climate sensitivity. As I understand it, the tropospheric hot spot would reflect the amplification of the co2 forcing by the positive feedback from water vapor that is assumed in the global climate models, through its effect on moist lapse rate. Regardless of how the data may be tortured, as in the "wind shear" argument cited above, all of the radiosonde data shows no evidence for a tropospheric hot spot. And without this tropospheric hot spot, the theory of water vapor multiplying the effect of higher c02 is disproven. Without higher climate sensitivity, the effects of increasing co2 on global temperatures are much more limited. Isn't this the real problem that the lack of evidence for a tropical hotspot presents for the anthropogenic global warming theory, ie. that the theorized high climate sensitivity to co2 increases will have positive feedback with large and catastrophic effects?

1) The lack of a tropospheric hotspot shows the absence of the lapse rate feedback - a negative feedback. The absence of a negative feedback does not show that there are no significant positive feedbacks.

2) The Water Vapour feedback depends primarily on water vapour concentrations in the lower troposphere which are known by direct observation to have increased. The situation where you have a significant increase in WV in the lower troposphere but not in the mid and upper troposphere would result in a strong WV feedback and a weak lapse rate feedback, thus resulting in an overall stronger net positive feedback from the effects of WV.

3) The total effect depends critically on the actual change in the WV concentration profile, which is not well established so the net final effect cannot be absolutely nailed down.

4) Paleoclimate based estimates of climate sensitivity make no assumptions about the relative contribution of different feedbacks, and still cluster around the 3-4 degree C range per doubling of CO2. It may well be that the net feedback of WV (WV feedback - lapse rate feedback) is weaker than predicted by the models, by the net ice and snow albedo feedback is much stronger (which is almost certain). Focusing on just one part of the equation is no basis for ignoring the paleo evidence on climate sensitivity.

Also note, that climate sensitivity isnt about CO2 forcing necessarily, but the sensitivity to any radiative forcing change. As TC notes, the water vapour feedback is established directly - some pretty fundamental physics would have to be wrong if not present. Your comments about "torturing data" are misplaced. The question is whether such measurements are able to show the model effect.

Finally, "large and catastrophic effects" is rhetoric. Since this is a science site, perhaps its better to confine discussion to effects actually predicted by the science? (ie AR4 WG2). "Catastrophic" means different things to different people. ie for some it would be forced-migration from large deltas because of salt-invasion; while for others it appears to be higher taxes or fuel costs. The argument is that evidence to date suggests its cheaper to mitigate emissions now, rather than pay cost of adaption later.

Responding to this post by tompinlb.
In addition to the discussion in the Advanced tab of this article, issues over errors and biases in radiosonde data are discussed here with references to the relevant papers.

Like scaddenp, my response to tompinlib would be to point out that there is plenty of reason to think that the problem may lie with the radiosonde observations as well as the models ("all models are wrong, but some are useful" - GEP Box). They were designed for use in weather forecasting, not climate modelling, and so while the data may have a resolution of 0.1K, they have many potential sources of bias which means that they are unlikely to be well calibrated. A lot of computer modelling work has to be done to homogenise the data to remove these sources of bias and it is ongoing work. The fact that there is a high degree of uncertainty in the radiosonde observations is well illustrated by the differences between radiosonde products from different research groups. See the advanced tab of the article.

Wouldn't the fact that the warming is in fact concentrated at the poles this past decade or so, and the low latitude ocean surface temps being dominated by a quiet ENSO so having not had a lot of warming lately (owing to the heat being carried into the deep ocean), make it reasonable that an effect that is ascribed to warming of the area SHOULD weaken when the area is not warming that much?

Just sayin... If one is looking for this in the tropical troposphere as something that is happening ALL the time one may be ignoring what is actually happening SOME of the time.

What is the significance of the hot spot signal? The hot spot is predicted by GCMs which predict strong net positive feedbacks in the climate response. This is why a doubling of Co2, which would all things being equal, cause 1.2C warming, can, via positive feedbacks purportedly lead to much more warming. Where is the hot spot? "Conversely, the data isn't conclusive enough to unequivocally say there is no hot spot." The burden of proof lies with those looking for the hot spot, not with the null hypothesis.

"Does this mean the greenhouse effect causes the hot spot? Not directly". If the climate system is highly sensitive to increases in Co2 radiative forcing, it must be fairly sensitivite to all forcing. The feedbacks are enacted more or less the same no matter what causes the warming right?

There does not seem to be strong evidence that the (long term?) hot spot exists. This is a problem for climate models which predict it. Of course more observations might reveal it, or they might not. For now, the evidence is not there.

"They also discovered that the results from RSS, NOAA, and the new study all show tropical amplification and are in agreement with the expected amplification from climate models. They state, “There is no significant discrepancy between observations and models for lapse rate change between the surface and the full troposphere.”